Linear low-density polyethylene (LLDPE) is produced by copolymerizing ethylene and alpha olefin using a polymerization catalyst under low pressure. Thus, this is a resin having a narrow molecular weight distribution and a certain length of a short chain branch, without a long chain branch. The LLDPE film has high strength and elongation at break in addition to the properties of a general polyethylene and exhibits excellent tear strength, falling weight impact strength or the like. This has led to an increase in the use of a stretch film, an overlapping films or the like which is difficult to apply existing low density polyethylene or high density polyethylene.
However, LLDPE is excellent in mechanical properties, but its blown film processability is poor. A blown film is a film produced by a method of blowing air into a molten plastic and inflating it, which is also called an inflation film.
Bubble stability, processing load and the like should be considered as factors to consider when processing a blown film. Especially, bubble stability should be considered important. The bubble stability means a property that when a film is produced by injecting air into a molten plastic, the produced film maintains its shape without being broken, which is related to the melt strength (MS).
The melt strength means strength for maintaining a shape that is resistant to molding and processing in a softened and melted state. The melt strength of low density polyethylene (LDPE) is higher than that of LLDPE. This is because, in the case of LDPE, branched chains are entangled with each other as compared with LLDPE, which is more advantageous in enduring the molding and processing. Thus, in order to supplement the melt strength of LLDPE, a method of producing a film by blending LDPE (low-density polyethylene) has been proposed. However, the above method has a problem of remarkably lowering the mechanical properties of conventional LLDPE even when LDPE is added in a very small amount.